Generally, metal halide discharge lamps are of the intermediate or relatively high wattage variety such as about 175 to 1500 watts for example. Also, it is known that the efficacy of the lumen output to input power decreases as the wattage of the lamp decreases. Thus, it has been generally presupposed that at lower wattages, wattages of 100 watts or less, metal halide discharge lamps would be entirely unsatisfactory in so far as efficacy is concerned.
Also, it has been a common practice in the intermediate and relatively high wattage lamps to provide an inert fill gas in the outer envelope in order to prevent oxidation of metal parts of the arc tube mount. Another advantage of an inert gas fill in an outer envelope is a high breakdown voltage which prevents arcing between metal parts of the arc tube mount. However, an undesired heat loss due to convection currents of the inert gas in the outer envelope reduces the lamp efficacy significantly.
One known attempt to reduce these undesired heat losses due to convection currents is disclosed in U.S. Pat. No. 4,499,396 to Fohl et al., issued on Feb. 12, 1985 and assigned to the Assignee of the present application. Therein, a quartz envelope is disposed within the gas filled outer envelope of a metal halide discharge lamp in an effort to reduce heat losses due to convection currents.
Another attempt to reduce undesired heat loss due to convection currents is set forth in U.S. Pat. No. 4,281,274. Therein, a glass cylinder surrounds an arc tube within an outer glass envelope. The outer glass envelope includes one or more lamp filaments and is filled with a gas under pressure. Thus, a glass cylinder and a gas filled outer envelope are employed to reduce the heat loss due to convection currents. However, structures having gas filled envelopes and accompanying convection currents leave something to be desired in reduction of heat loss in so far as relatively high pressure lamps are concerned.